Magnetic recording devices, such as magnetic disk and tape drives, use heads to read and write information to and from a magnetic surface. In a typical rotating storage device, data is stored on magnetic disks in a series of concentric tracks. These tracks are accessed by the read/write head, which detects variations in the magnetic orientation of the disk surface. Note that a track generally refers to a segment parallel to relative motion of the head. The read/write head moves back and forth radially on the disk under control of a head-positioning servo mechanism so that the head can be selectively positioned over a specific track.
In more detail, an actuator arm typically moves the head across the data tracks under the control of a servo system based on servo data stored on the disk surface within dedicated servo fields. The servo fields can be interleaved with data sectors on the disk surface or on a separate disk surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system rotates the actuator arm to adjust the head's position so that it follows the center line of the selected track.
One parameter that impacts the overall storage capacity of such rotating storage devices is referred to as track density (e.g., tracks per inch). The greater the track density, the greater number of tracks that can be recorded on the disk, thereby providing greater overall data storage capacity. A known factor limiting track density is referred to as adjacent track interference.
In particular, as track density increases, the information from several tracks will appear mixed together on the readback head, thereby degrading signal integrity. Thus, even though write heads are technically capable of writing into smaller areas (thereby enabling the dense packing of a storage disk), the available read heads tasked with extracting the stored data have limitations that restrict the size of the storage cells and track spacing.
To avoid this adjacent track interference so as to maintain data integrity, conventional disk drives are required to write the tracks sufficiently far apart (lateral separation) to effectively reduce the amplitude of the adjacent interfering tracks. Guard bands are used to separate neighboring tracks. No usable data can be stored in the guard bands. A single readback signal is used in retrieving the stored data from the tracks of the disk.
Readback intelligibility (i.e., data integrity) is therefore achieved by spatially separating the magnetic information on the disk. This required separation of tracks on the disk limits the track density of the drive for a given disk size and geometry. Moreover, storage capacity is fundamentally limited and can only be increased by increasing the size or number of layers on the disk.
What is needed, therefore, are techniques for increasing the readback intelligibility for densely packed disks. In a more general sense, there is a need for techniques that increase storage capacity without having to increase the size of the disk or of the disk drive.